In a power supply for stably supplying electric power to electronic devices, higher reliability thereof is required while a capacity thereof is continuously increased. In such a power supply, it is important not only to stably supply electric power required in a system, but also to minimize a necessary space. Therefore, in order to minimize the space required by the system, it is necessary to reduce sizes of unnecessary elements such as a heat sink by increasing power conversion efficiency while making the power supply smaller, lighter and thinner. Generally, a switching power supply has low power conversion efficiency due to a switch loss, and thus various power conversion techniques for improving the power conversion efficiency have been developed.
For example, a resonance type power converter is a power converter having high efficiency even in a high frequency range by making a voltage or a current of a switch zero at the moment of switching and minimizing the switching loss. In the resonance type power converter, a resonance switch may be constituted by adding a capacitor and an inductor which cause resonance to a switch element.
The resonance type power converter initially started to be applied to DC power supplies, inverters or the like in aviation and space fields which require a small size and a light weight, and the application thereof has recently extended to industrial fields. Such a resonance type power converter is classified into a series resonance type converter and a parallel resonance type converter according to a resonance type. A quasi-resonance type converter and a multi-resonance type converter having a small number of switches are used in a small-sized power supply. A resonance type inverter with a resonant link has been studied for an AC power supply and is applicable to a power system requiring a high frequency. Meanwhile, a soft switching converter of a pulse width modulation (PWM) converter type which minimizes resonance energy by performing zero voltage or zero current switching only during switching is also being studied.
A switch constituting such a power converter is a switch element as a semiconductor element, and a loss occurs due to an emergency characteristic of the switch element when the switch element is turned on and off.
FIG. 1 is a waveform diagram illustrating a power loss of a switch element.
Referring to FIG. 1, a power loss PL of a switch element is increased, as the turn-on time ton and the turn-off time toff of the switch element become longer and also increased in proportion to a switching frequency. However, in order to reduce a size and a weight of a power supply, a switch element of a power converter is generally switched at a high frequency, but the high-frequency switching of the switch element increases a loss due to the switching. The switching loss has a problem of lowering the power conversion efficiency. Also, in a power conversion system such as new renewable energy and an electric car, bidirectional power conversion is required. At this time, bidirectional power control is implemented by one power stage to simplify the system and also to reduce a cost thereof. However, due to a leakage inductance between a primary side winding and a secondary side winding of an insulation transformer included in the bidirectional power conversion system, when the high frequency switching is performed, the switching loss is increased and a surge voltage is generated. Therefore, in the power converter, there is a problem that the power conversion loss and a voltage stress applied to the switch are increased.